Heart disease continues to be the leading cause of death in the United States and is associated with a variety of clinical abnormalities, including congestive heart failure, ventricular remodeling, ischemia, ion homeostasis, arrhythmias, and sudden cardiac death. The hypotheses motivating this proposal are that metabolic factors play a role in heart disease through their tight interrelationship with cardiac electrophysiology (CEP) and that identification of the above-mentioned phenomena will strengthen our mechanistic understanding of rate-dependent cardiac remodeling, heart failure, and fatal arrhythmias, as well as lead to new therapies. Advanced technologies are necessary to generate and test new hypotheses, particularly in the isolated rabbit heart, which has been a key model for whole-heart electrophysiology. The challenge in developing new treatments for heart disease is the ability to correlate spatial patterns of metabolism with those of electrical activity associated with spatial heterogeneities (e.g., regional ischemia), and potentially fatal arrhythmias - an ability that would enable the testing of hypotheses regarding metabolic interventions to reduce the risk of fatal arrhythmias. Hence we propose to implement technological innovations that will allow the integration of our state-ofthe-art electrophysiological imaging of the isolated rabbit heart with optical imaging of the metabolism associated with both regional ischemia and tachycardia, i.e., to create correlative Multimodal Cardiac Imaging (MCI). The project involves comparing and validating several complementary modalities for metabolic measurements in the isolated rabbit heart, and correlating these with our previously validated electrophysiological measurements of the spatiotemporal dynamics of cardiac activation, propagation, recovery, reentry, and defibrillation. Aim 1 is to develop and characterize simultaneous metabolic and electrophysiological imaging techniques by: a) developing simultaneous dye-based fluorescence imaging of transmembrane potential, Vm;cytosolic calcium, [Ca2+]i;extracellular potassium, [K+]e;and reactive oxygen species, ROS;and ultraviolet autofluorescence measurement of reduced nicotinamide adenine dinucleotide, NADH;b) using biochemical assays to independently quantify the temporal variation of the cardiac metabolic state;c) integrating these new imaging modalities with our existing electrophysiological instruments, and extending the approach to allow the study of regional hyperkalemia, hypoglycemia, anoxia, and ischemia. Aim 2 is to quantify the bidirectional coupling between cardiac metabolism and arrhythmogenesis by: a) studying how regional hyperkalemia, hypoglycemia, anoxia and ischemia affect arrhythmias;b) determining the metabolic responses to arrhythmias and acute andchronic rapid pacing;c) correlating the metabolic observations with electric, optical, and magnetic measurements of action potentials, action and injury currents, membrane capacitive and ionic currents, and stimulation currents;and d) determining if cardiac metabolic state affects the response of the heart to point and defibrillation-strength field stimulation. This work will be a key step towards developing improved metabolic therapies.